The use of composite structures for various components of modern aircraft is expanding. Increasingly, large portions of an aircraft's structure may be manufactured from composite material, including wings and fuselage skins, certain substructures, aerodynamic surfaces, and engine/exhaust components. The benefits of using composite structures include high strength to weight ratio, corrosion resistance, thermal stability, and long fatigue life. In a number of applications it is desirable for certain composite structures that are subjected to continuous or transient operating conditions in the temperature range of between 550-750.degree. F. to be fabricated from so-called high temperature materials. Such high temperature materials are preferable in engine or exhaust structural or fairing components used in commercial and military aircraft and missiles, and aerodynamically heated components of supersonic aircraft. Additional applications for high temperature materials include high temperature exhaust impinged components such as wing flaps on certain aircraft, engine areas of spacecraft, solar-heated components, or any exhaust-washed structural component.
Typically, these composite structures, especially when used on edge components, are susceptible to small area damage during operational use from items such as stones, hail, or shrapnel. Additionally, damage may occur from maintenance workstands and dropped tools while the aircraft is undergoing maintenance activities. A continuing desire of those skilled in the art is to develop methods to repair the composite structures to conform both structurally and electrically to the originally fabricated component.
Although repair processes for more common low temperature composite components are widely available (such as for 250.degree. F. epoxies, 350.degree. F. bismaleimides, and 550.degree. F. polyimides), many of these techniques suffer from significant disadvantages and are not easily applicable to higher temperature composites. For example, because of the high temperatures required to cure the composite materials, damage may occur to the portion of the structure surrounding the repair area during the long, high temperature cure cycle typically required for high temperature materials. The alternative method of using localized heating (for example, a heating blanket) generally requires the use of applied pressure, such as by using a vacuum bag. However, there are concerns of survivability of bagging materials in high temperature, potentially oxidative environments. Traditional breather/release films are glass type materials coated with TEFLON.RTM. polymer and are known to decompose in oxidative environments above 650.degree. F. Also, chemical release agents, such as, for example, FREKOTE.RTM. 33 or 44 (available from Dexter Corporation, Seabrook, N.H.) are limited to temperatures of 700.degree. F. In addition, bagging films such as KAPTON.RTM. (manufactured by DuPont), UPILEX.RTM., and other polyimide-based films are generally restricted to 700.degree. F. Heat blankets typically overlap undamaged adjacent structure, which can cause collateral damage during the cure process. Another difficulty encountered in repairing composite structures is the need to consistently and accurately position the repair plies to ensure the structural and electrical integrity of the repaired component.
Accordingly, there is a continuing need for a method to repair high temperature organic matrix composite structures. Such a method could desirably repair both the structural and electrical integrity of the original structure. Ideal repair processes for such high temperature composites would have certain characteristics including, a reduction in the total repair time minimizing the use of special support equipment, the amount of consumable products used, and the amount of repair material used. Additionally, the method would prevent disruption of the structural integrity of the repaired component, ensure electrical compatibility of the repaired section to the parent structure, and would incorporate simple procedures thereby decreasing the skill requirements needed for the repair technician.
In order to fulfill these requirements, the method of the present invention uses a combination of unique repair preparation techniques, repair material processing methods, high temperature curing equipment applications, and high temperature vacuum bagging processes, to accommodate the high cure temperature and long cure cycle of high temperature materials. Additionally, the unique repair processes disclosed herein integrate multiple methods for restoring both structural and electrically absorptive properties.